Scholarly article on topic 'Bioelectro Chemical Systems: A Sustainable and Potential Platform for Treating Waste'

Bioelectro Chemical Systems: A Sustainable and Potential Platform for Treating Waste Academic research paper on "Chemical sciences"

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{Bioremediation / "microbial fuel cell (MFC)" / "bio-electrochemical system (BES)" / Bioremediation / biogas / "biomethanation ;"}

Abstract of research paper on Chemical sciences, author of scientific article — Sandipam Srikanth, Manoj Kumar, M.P. Singh, B.P. Das

Abstract Rapid urbanization and industrial growth across the globe has increased the generation of solid, liquid and gaseous wastes. This is leading to the contamination of soils, groundwater, surface water and air with hazardous and toxic chemicals. It is predicted from the’business-as-usual’ projections that the solid-waste generation rates will exceed 11 million tonnes per day by 2100, which is three times more than current rate. On the other hand, the increased wastewater production and its direct discharge into rivers is causing major usable water resource crunch. Carbon dioxide from industrial sources is also ultimate waste product which is major contributor to the climate change. In general, the technologies for disposal of solid, liquid and gaseous waste are less efficient and energy intensive. In the recent years a paradigm shift has taken place towards the outlook of waste disposal and thrust is shifting to use it as a resource for production of energy and commodities. Some of the widely practiced interventions in this direction may be cited as utilization of organic waste and sewerage water for production of methane as a fuel. Although the net energy yields in these conversion processes are generally low but many rapid advancements taking place to overcome these limitations. Bioelectrochemical systems (BES) are emerging as an exciting platform to convert chemical energy of organic wastes into electricity or hydrogen or value added chemical commodities. In BES, specific group of electro-active bacteria can be used as catalyst. Compared to traditional treatment-focused, energy-intensive environmental technologies, this emerging technology offers a sustainable solution for integrated waste treatment and energy and resource recovery, because it offers a flexible platform for both oxidation and reduction reaction oriented processes. In this way, BES works on interface of fermentation and electrochemistry. The applications of BES into waste treatment domain may include, current generation, efficient bioremediation of a wide range of organic wastes, desalination, color removal, toxicity reduction, gaseous pollutants treatment and synthesis of commercially viable chemicals and solvents from CO2 reduction. This paper discusses the current state of the technology and emerging innovations on BES application with respect to solid waste management, wastewater treatment and CO2utilization, including the on-going research at IndianOil R&D centre.

Academic research paper on topic "Bioelectro Chemical Systems: A Sustainable and Potential Platform for Treating Waste"

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Procedia Environmental Sciences 35 (2016) 853 - 859

International Conference on Solid Waste Management, 5IconSWM 2015

Bioelectro Chemical Systems: A Sustainable and Potential Platform

for Treating Waste

Sandipam Srikantha, Manoj Kumarb, M.P. Singhc, B.P. Das1^*

a Research Officer, Indian Oil Corporation Limited, Sector 13 Faridabad, Haryana, India b Deputy Manager Research, Indian Oil Corporation Limited, Sector 13 Faridabad, Haryana, India c Chief Research Manager, Indian Oil Corporation Limited, Sector 13 Faridabad, Haryana, India dExecutive Director (R&D), Indian Oil Corporation Limited, Sector 13 Faridabad, Haryana, India

Abstract

Rapid urbanization and industrial growth across the globe has increased the generation of solid, liquid and gaseous wastes. This is leading to the contamination of soils, groundwater, surface water and air with hazardous and toxic chemicals. It is predicted from the 'business-as-usual' projections that the solid-waste generation rates will exceed 11 million tonnes per day by 2100, which is three times more than current rate. On the other hand, the increased wastewater production and its direct discharge into rivers is causing major usable water resource crunch. Carbon dioxide from industrial sources is also ultimate waste product which is major contributor to the climate change.

In general, the technologies for disposal of solid, liquid and gaseous waste are less efficient and energy intensive. In the recent years a paradigm shift has taken place towards the outlook of waste disposal and thrust is shifting to use it as a resource for production of energy and commodities. Some of the widely practiced interventions in this direction may be cited as utilization of organic waste and sewerage water for production of methane as a fuel. Although the net energy yields in these conversion processes are generally low but many rapid advancements taking place to overcome these limitations.

Bioelectrochemical systems (BES) are emerging as an exciting platform to convert chemical energy of organic wastes into electricity or hydrogen or value added chemical commodities. In BES, specific group of electro-active bacteria can be used as catalyst. Compared to traditional treatment-focused, energy-intensive environmental technologies, this emerging technology offers a sustainable solution for integrated waste treatment and energy and resource recovery, because it offers a flexible platform for both oxidation and reduction reaction oriented processes. In this way, BES works on interface of fermentation and electrochemistry. The applications of BES into waste treatment domain may include, current generation, efficient bioremediation of a wide range of organic wastes, desalination, color removal, toxicity reduction, gaseous pollutants treatment and synthesis of

* Corresponding author.

E-mail address: dasbp@indianoil.in

1878-0296 © 2016 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of 5IconSWM 2015

doi:10.1016/j.proenv.2016.07.102

commercially viable chemicals and solvents from CO2 reduction. This paper discusses the current state of the technology and emerging innovations on BES application with respect to solid waste management, wastewater treatment and CO2utilization, including the on-going research at IndianOil R&D centre.

© 2016PublishedbyElsevier B.V. Thisisanopenaccess article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of 5IconSWM 2015

Keywords: Bioremediation, microbial fuel cell (MFC), bio-electrochemical system (BES); Bioremediation, biogas, biomethanation;

1.0 Introduction

The advent of new century has witnessed extensive industrial growth and urbanization resulting in increased generation of solid, liquid and gaseous wastes. Consequently, there is alarming environmental pollution like accumulation of enormous amount of solid waste, contamination of water resources due to manmade emissions and has evidenced a a rise in global warming and has evidenced a rise in global warming. It is predicted from the 'business-as-usual' projections that the solid-waste generation rates will exceed 11 million tonnes per day by 2100, which is three times more than current rate (Hoornweg et al., 2013). On the other hand, the increased wastewater production and its direct discharge into rivers is causing major usable water resource crunch (van Loosdrecht and Brdjanovic, 2014). The waste management is becoming a serious global issue. However, nations are trying to reduce the amount of waste generation and governments also adopting various policies for waste management, and are creating instruments for incentives to various players involved in waste management. But most of the countries are still treating these wastes in a conventional manner as decades ago. Considering all these major issues, innovative technologies are needed to develop for the solid waste management as well as wastewater treatment and CO2 mitigation.

Commonly used technologies for treating various kinds of waste streams include thermo-chemical digestion and bio-assisted methods. The technologies for solid waste disposal as well as liquid and gaseous waste treatment are, in general energy intensive and adversely effect the environmental integrity of science. Most of these processes release CO2 into the environment, causing increased global temperatures. Moreover, in the recent years a paradigm shift has taken place in the waste disposal towards conserving the energy stored in the waste organics. This has put forward the necessity of rapid evolution of bioenergy discipline to solve the looming energy crisis as well as to save the planet from the brink of an environmental catastrophe. Biofuels/bioenergy production from waste resources opens windows for an exciting and sustainable alternative to the fossil fuels which can help in the worldwide energy crisis and environmental pollution problems. In this direction, few strategies were developed towards energy recovery from waste organics, such as biomethanation and biohydrogen production. However, the efficiency of these processes in terms of waste recycling and energy recovery is not upto the mark due to the inability in dictating the microbial metabolism, especially with mixed consortia that suits for the waste organics. In this realm, a novel system with selective microbial process towards designated it as bio-electrochemical system (BES)

2.0 Principles and advantages of BES

BES works at the nexus of water/waste and energy, representing a new and promising biological process for energy recovery and generation (Rabaey and Rozendal, 2010). This trans-desciplinary system is based on electro-active microbes that are capable of exchanging electrons with electrodes. BES works on interface of fermentation and electrochemistry as base research areas, although, various other sciences like electrode technology, material sciences, separation technology, engineering also contribute to make the system complete. The applications of BES include, current generation, bioremediation of a wide range of wastes, specific pollutants/xenobiotics removal, desalination, color removal, toxicity reduction, gaseous pollutants treatment, synthesis of commercially viable chemicals and solvents, CO2 reduction, etc. BES holds a great promise for sustainable production/recovery of energy and chemicals.

BES can be classified into various categories based on their application

Microbial fuel cell (MFC) for power generation from waste remediation (Figure 1) Microbial electrolysis cell (MEC) for biofuels production from waste streams in electrolysis mode Microbial desalination cell (MDC) for desalinating saline water along with power production Mcrobial electrosynthesis (MES) for synthesizing the commodities (fuels/chemicals)

Waste/ Wastewate

H+ - * -5 o

■5 u

Fig. 1. Schematic representation of a typical MFC

MFC facilitates direct transformation of chemical energy stored in the bio-convertible substrate to electrical energy via microbial catalyzed redox reactions (fermentative) involving microorganisms as biocatalysts under ambient temperature/pressure in absence of oxygen. The electron (e-) present in reduced substrates (e.g., carbohydrates, organic acids, H2, S2-, CH4) begin an "electric circuit" that ends when these electron reaches the e-sink (mostly oxygen) furnished by a final electron acceptor through electrodes (Venkata Mohan et al., 2014). In simple words, the oxidation reaction (generating the reducing equivalents) separated from its terminal reduction reaction by a selectively permeable ionic membrane (generally proton exchange membrane) allows to capture the e-by a hybrid system called microbial fuel cell (MFC), a type of BES (Venkata Mohan et al., 2014). Similarly, if the energy generated in the system is used to reduce the protons released to form H2 or CH4, that system is called as MEC. If the energy is used to desalinate the saline streams in a three chamber mode through exchange of ions across anion and cation exchange membranes, that is known as MDC. A recent innovation of using the energy generated in the BES is synthesis of commodities, known as MES. MES could be considered as an effective strategy to store the electrical energy, in the form of chemical structures.

A key advantage of BES is that electric power or products can be generated from renewable and carbon-neutral waste materials, with consequent lower greenhouse gas (GHG) emissions compared to conventional technologies. The unique control of BES enables real-time monitoring of the biocatalytic process. The other advantage of these systems is their high selectivity and high speed. BES accelerates the reaction kinetics, so that the designated reaction yields would be more than conventional processes. In this process, the in situ generated potential of the system or the applied small voltages will help the biocatalyst in overcoming the thermodynamic energy barrier to form the respective product. Negative valued waste can be used as substrate to generate energy or fuels in BES due to the fact that the system provides the necessary electrons required for the reaction.

Utilizing wastewater or solid waste as anodic fuel in BES has a dual advantage of energy generation with simultaneous treatment. BES when operated with wastewater composed of diverse components, some components act as electron acceptor either in anode between biocatalyst and electrode or in cathode chamber and get reduced to

their respective end products. For example, nitrate rich wastewater can be treated at cathode by providing conditions so that the nitrates will act as TEA. Similarly, sulphates and other organic and inorganic pollutants can be treated by providing the requisite electron acceptor conditions. However, the synergistic interaction between the MFC/BES components and the biocatalyst needs to be understood and optimized to fully exploit the capacities of these systems in order to maximize the product recovery and energy generation (Srikanth et al., 2011).

2.1 Technology status

Various waste organics as well as wastewaters from different origins containing different kinds of organic constituents were studied as substrates for BES. However, the nature and composition of waste determines its efficiency of BES in terms of power generation. Solid waste contains low water content and high solids/organic matter which are not amenable for treatment. Based on solids concentration, either it will be diluted with water or domestic sewage or pretreated to get the leachate before subjecting for biological treatment. Similarly, wastewater also divided into high and low biodegradable, based on the nature and concentration of their constituents. Still, the wastewater could be a potential carbon source for MFC due to the possibility of converting negative valued waste into energy.

2.1.1 Solid waste treatment

Solid waste from various sources viz., Food industries (ElMekawy et al., 2014a; Goud et al., 2011), marine environment (Bond et al. 2002), agriculture and domestic sectors (ElMekawy et al., 2014a), waste activated sludge (Asztalos and Kim, 2015), municipal waste (Kook et al., 2015), petroleum based oily sludge (Chandrasekhar and Venkata Mohan, 2012) etc., were used as potential feed-stocks for BES.

Agricultural residues like corn stover, cattle manure, wheat straw, etc. have been tried as substrate in BESs. On the other hand, highly biodegradable food wastes in the form of vegetable waste, canteen based waste, yogurt waste, and other edibles are available in surplus. These can be readily used to tap bioelectrochemical energy due to their rich organic content. Muncipal solid waste (MSW) is another major aspect of solid waste which is accumulating in huge quantities everyday and has a possibility to use as fuel in BES. Among the solid wastes studied, food industry based wastes are more studied. Chandrasekhar and his co-workers studied the degradation of canteen based food waste (chemical oxygen demand (COD), 380 g/l) in solid state fermentation mode using a cylindrical single chambered MFC with air-cathode (Chandrasekhar et al., 2015). Anaerobic mixed culture was used as inoculums and food waste collected from canteen was used as substrate. The oil content (38 g/l) was removed by gravimetric separation prior to feeding into reactor and added with 10% tap water to maintain moisture content. Maximum power density of 164 mW/m2 was observed along with a hydrogen production rate of 21.9 ml/h followed by ethanol production at 4.85% (w/v). As the waste contained huge biodegradable organic content, there is a possibility of 3 different energy productions along with a substrate removal of 72% in terms of COD. This shows the strong potential of BES in energy generation as well as conservation into fuels/chemicals. In another study by Kook and his co-workers, using liquid fraction of municipal solid waste as substrate, about 94% COD removal was observed in association with a current density of 152-218 mA/m2 (Kook et al., 2015).

2.1.2 Wastewater treatment

Wastewater from food based industries, domestic origin and agro industries contains higher contents of biodegradable organic fraction that can be treated efficiently (ElMekawy et al., 2014a). On the other hand, the wastewater from chemical, pharmaceutical, metal and refinery based industries are having low biodegradable organic fraction that tends to have lower treatment efficiency. Various wastewaters from diverse origins have been studied as substrate in BES.

Wastewater streams from food processing and beverage industries like brewery, winery, dairy, vegetable, meat, and other food-processing industries are abundant in availability, rich in organic content, and possess high biodegradability(Digman and Kim, 2008; ElMekawy et al., 2014a). The effluents from slaughter houses, chemical and brewery industries are the most sought out substrates in BESs among the low biodegradable wastewater (Katuri

et al., 2012; Min et al., 2005). The ease of availability and the necessity to treat these high organic matter-containing high strength effluents have made these wastewaters an ideal fuel source for BES. Complex slaughterhouse based wastewater showed about 93% COD removal along with a power density of 578 mW/m2 in BES (Katuri et al., 2012). When domestic wastewater was used as substrate in BES, power density of 155 mW/m2 was observed along with about 67% COD removal (Mohan et al., 2009). Similarly, when BES was integrated with fermentation system as secondary unit, it showed about 165 mW/m2 power density along with 72% COD removal (ElMekawy et al., 2014b). In extension to the MFC studies, the energy generated in the system is directly converted to generate gaseous fuels in an electrolysis mode in MEC, where by controlling the potential difference or by supplying the external electrons, a support will be provided to the microbes to cross the thermodynamic barrier of the reaction resulting in product synthesis (Venkata Mohan et al., 2014). In detail studies related to MEC were reported in the direction of H2 and CH4 (electro-methanogenesis) production. Especially, when complex or solid wastes were used as substrates for methane generation, application of BES in electrolysis mode enhanced the product yield as well as degradation (Sasaki et al., 2010; Villano et al., 2010; Yan et al., 2013).

2.1.3 CO2 Utilization

Very recently, the CO2 capture and its utilization became an attractive technique for the reduction of ever increasing carbon foot prints on earth, as well as for complementing the bioenergy/biofuel sector with the synthesized products (Barbarossa et al., 2014). CO2 reduction in BES is similar to naturally occurring photosynthesis, when it is coupled with the energy generated from renewable sources like wind and solar systems (Figure 2).

In contrary to electrochemical methods, sufficiently low electrode potentials can be created in bioelectrochemical system for CO2 reduction by utilizing biocatalysts that possess the CO2 fixing metabolism. In the bio-electrocatalysis, unlike the conventional and photo-electrocatalysis, very less energy input is required to help the biocatalyst in crossing the energy barrier. This way the bio-electrocatalysis process is more eco-friendly and less energy intensive, which can be carried out with both microbes and enzymes. As the technology is at its infancy, only few studies have been reported with their preliminary results and an extensive research is going on in this direction. First study in this area has been published in 2010 by Nevin and his co-workers, where they proposed the electron uptaking capability of Sporomosa Ovata (Nevin et al., 2010), which led to a new application of BES in product synthesis from CO2. After this, several researchers across the globe started working in this area towards developing a complete platform for closing the carbon cycle at faster rate. In a recent study, the enriched homoacetogenic mixed consortia showed aout 4 g/l of acetate production using bicarbonate as substrate (Mohanakrishna et al., 2015). In a similar study with enriched mixed culture biofilm, direct CO2 was used as substrate and reduced to acetate (1.5 g/l) (Patil et al., 2015). On the other hand enzyme based CO2 capture is also being studied in BES, where the CO2 is converted into formic acid (~0.4 g/l) using formate dehydrogenase at cathode (Srikanth et al., 2014).

3.0 Future outlook

Though, an extensive work is being carried out in BES field, there are certain constrain still to be addressed to make the process economically viable and commercially sustainable. Application of BES for the valorization of solid waste is started very recently, which needs a core focus, especially in the direction of utilizing complex waste organics, such as oily sludge, cellulosic biomass, waste activated sludge, etc., which are very difficult to treat conventionally but necessarily be treated for the benefit of mankind. In the wastewater segment also, very few reports available in relation to the high strength wastewater, which should essential be considered for further studies. Complex wastewaters treatment such as distillery and refinery based wastewater should be addressed using BES, which are having large potential for up-scaling. In the direction of CO2 mitigation, various products like acetate, methane, formic acid, butyrate, oxo-butyrate, methanol, ethanol, etc., were reported using BES. However, considering the limitations, viz., enrichment of selective microbes, biofilm development on cathode, synergistic interaction of the microbes with electrodes, electron uptaking ability of microbes, other operating, physic-chemical and biological factors, etc., extensive research needs to be put in to understand the process as well as to commercialize it in the near future.

Fig. 2. Schematic representation of a typical MES process

4.0 IOCL research initiatives in the area of BES

IOCL R&D center has initiated its research in the area of BES with the objectives of solid waste management, wastewater treatment and CO2 mitigation. As part of this, we are working on electro-methanogenesis aspect with solid waste and biomass residues, where the major goal is to enhance the biogas yields as well as purity of the product. Besides that, efforts are being put for the BES based treatment of refinery wastewater coupled with energy generation. Refinery wastewater contains different complex constituents, such as oil & greese, sulfides, phenolic compounds, etc., which are difficult to treat. Major focus of this project will be to handle such type of constituents with simultaneous enhancement in power output. In the area of CO2 mitigation also, IOCL R&D started working extensively, with a research focus on conversion of CO2 to fuels and hydrocarbons.

5.0 Conclusions

Recently BES application in the field of waste management and CO2 mitigation has shown very promising outputs. However, it is still away from commercial exploitation. There are needs to be further perused in terms of optimization of conditions for additional treatment efficiency along with power generation or product formation. Though, the process is understood well, still there are certain limitations pertaining to its scale-up. Significant efforts are needed in this direction to increase the potential of BES in the wastewater treatment and bioenergy sector. Further to that, difficult biodegradable wastes such as refinery industry based waste streams as well as industrial offgases are need to be targeted as substrates for BES, where there is a lot of scope in terms of environmental protection and sustainable process developments. Metabolic engineering through application of molecular and genetic tools will provide the designed microbes for the accelerated remediation and energy generation processes. Beside that innovative engineering principles need to be applied for up scaling of BES system.

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